well, what everyone is saying is still card him anyway, at the next break in play.
This!!
There seems to be a misunderstanding here. A card is not a sanction, its a tool for the referee to deal with players who commit acts of extreme dangerous or foul play, or who seem to be ignoring previous sanctions. You don't have to give the card at the time you whistle for the PK. You don't even have to even award a PK at all to award a card. Here is a real game scenario I have described before, for those who haven't read it.
lBue v Gold. Blue breaks down left hand side, and about the Gold 22, the Blue left winger throws an inside pass to his centre, who goes around and scores under the poles. Unseen by referee. Gold right winger takes out Blue left winger with a very late, high swinging arm. This was spotted by the TJ* who hangs his flag. After consultation between referee and TJ*, the referee RCs the Gold right winger and the try stands.
* Yes, TJ. This was a Super 12 match, Highlanders v Hurricanes circa 1998/1999/2000. This was pre-AR and pre-TMO involvement in anything outside of in goal. IIRC the referee was Paul Honiss.